1. Field of the Invention
The object of the invention is a signal receiver and a method for optimizing gain of a tuner.
2. Brief Description of the Background of the Invention Including Prior Art
A common problem during the signal reception, especially the reception of terrestrial television signals, is related to strong undesired signals close to a weak desired signal. The strong signals may be generated by a transmitter, which is closer than the transmitter of the desired signal. There are several known methods for solving this problem, but they are applicable for specific receivers only.
There is known from the U.S. Pat. No. 6,178,211 “Digital processing radio receiver with adaptive bandwidth control” a method for processing an intermediate frequency signal, in which the filter characteristic is narrowed in order to eliminate the interferences from the adjacent signals. The method can be used only in receivers, which allow controlling the width of the filter characteristic.
There is known from the European patent application No. 0903937A2 “Digital television signal receiving tuner system” a dual frequency conversion tuner, in which the first intermediate frequency filter attenuates the signals close to the received signal. However, it attenuates only signals within the passband of the first IF filter, and it does not protect against the overload of the input stages of the tuner by signals outside the passband of the IF filter, but within the passband of the input filter.
There is known from the European patent application No. 1398930A1 entitled “Radio-frequency-signal-receiver and method of manufacturing the same” a method for optimizing the gain of a tuner, in which the input stage may be disrupted by strong signals close to a weak desired signal, where the AGC characteristic of the tuner is adjusted to compensate the effect of strong close signals. However, to determine the level of close signals, a distinct peak power detector is used as an additional component of the tuner, the detector band being limited by a filter. Such solution is applicable only to custom-designed tuners.
The present invention is designed for single or dual frequency conversion tuners, especially that in form of an integrated circuit chip, in which the width of the passband of the last intermediate frequency filter is narrower than the width of the passband of the high frequency input filter. An exemplary structure of a dual frequency conversion tuner is shown in FIG. 1. The tuner can be implemented as a single integrated circuit chip or can be comprised of several separate circuits. The tuner receives an input signal RF_IN, which is filtered by a band-pass high frequency input filter 101 (which may consist of a set of wideband filters). The filtered input signal is amplified by a Variable Gain Amplifier (VGA) 102, whose gain is controlled by RF_AGC signal, generated by a demodulator. The first mixer Mixer_1 103 converts the signal to a first intermediate frequency (IF). The mixed signal is filtered by a band-pass first IF filter SAW1 104, whose passband width is narrower than the passband width of the input filter. The signal is next converted by the second mixer Mixer_2 105 to a second intermediate frequency. Next the signal is input through a Fixed Gain Amplifier (FGA) 106 to a second IF filter SAW2 107. The signal from the SAW2 filter 107 is amplified by a second IF amplifier IF_Amplifier 109 with its gain controlled by IF_AGC signal, and next, the signal is output to the demodulator. The structure of a single frequency conversion tuner differs from that shown in FIG. 1 basically in that it does not comprise the elements shown in the dotted line, i.e. the first mixer 103 and the SAW1 filter 104, therefore it utilizes the intermediate frequency signal of a single frequency. The common characteristic of these two tuner types is that the width of the passband of the input filter 101 is larger than the width of the passband of the intermediate frequency filter SAW2 107.
The tuners of such structure may have a problem in reception of signals, close to which, within the passband of the input filter, are other stronger signals. The problem has been illustrated in FIG. 2. The RF_AGC and IF_AGC gains are set such that a signal of a constant level, required by a demodulator, is present on the IF_OUT output. The RF_AGC and IF_AGC correction is based only on the level of the signal within the passband of the last IF filter, and it does not include all signals that are within the passband of the input filter 101. Therefore, if the tuner gains were adjusted to the level of the weak signal S1, this could lead to tuner overload by a strong signal S2. The known solution to this problem is to set such AGC characteristic of the tuner that will decrease the gain of the VGA 102 amplifier, and concurrently increase the gain of the IF_Amp 109 amplifier. However, since the AGC characteristic in the known solution is constant, it requires a compromise. If the AGC characteristic would strongly limit the RF_AGC gain, it would be impossible to receive weak signals. If it would allow high RF_AGC gains, the tuner could be overloaded by strong signals close to the desired signal.